Move API credentials to .env and set gpt-5.6-luna model

- Load OPENAI_API_KEY and OPENAI_BASE_URL from gitignored .env (repo-root, CWD-independent)
- Fix command explanation to use the configured model instead of hardcoded gpt-4o-mini
- Update default model and fallbacks to gpt-5.6-luna
- Document .env-based configuration in READMEs

edison/ui/console.py

@@ -132,7 +132,7 @@ def handle_command_execution(client, config, command, explain=False):
             use_rich = ui_config.get("rich_formatting", True)
             
             # Generate explanation
-            explanation = get_command_explanation(client, command, structured=structured)
+            explanation = get_command_explanation(client, command, structured=structured, model=config.get("model"))
             
             if use_rich:
                 print()  # Add a blank line before the explanation